Vitamin D: Benefits, Deficiency, and How to Get Enough
- , by SANUSq Research team
- 13 min reading time
Vitamin D is one of the most talked-about nutrients of the last decade — hailed as a fix for everything from colds to cancer. The truth is more measured, and more useful: it's genuinely essential, widely lacking, and worth getting right — without believing the hype. This guide sets out the vitamin D facts that hold up, the vitamin D myths that don't, and what to do about it in practice.
What vitamin D actually does — and why vitamin D is important
Vitamin D isn't really a vitamin at all — it behaves like a hormone, and receptors for it sit on cells throughout the body, which is why it influences so many systems. Its best-established job is managing calcium: it helps you absorb calcium from food and lay it down in bone, which is why severe deficiency causes rickets in children and soft, weak bones in adults. Beyond the skeleton, vitamin D helps regulate the immune system, supports muscle function, and is involved in mood and brain health. That breadth is where the importance of vitamin D genuinely lies — but it's also why vitamin D gets over-claimed, so it's worth separating the solid from the speculative.
The sunshine problem: why deficiency is so common
Most of our vitamin D isn't eaten — it's made in the skin when UVB sunlight hits it. That elegant system has a catch: modern life gets in the way. Living at higher latitudes, spending days indoors, covering up or using sunscreen (sensible for skin-cancer prevention), having darker skin (more melanin means less vitamin D made for the same sun), and getting older (skin makes less with age) all reduce production. The result is that an estimated one billion people worldwide have insufficient or deficient vitamin D levels — making it one of the most common nutritional shortfalls there is. Even vitamin D in summer isn't guaranteed: a day spent indoors, covered up or under a high factor of sunscreen can leave the skin making very little at the one time of year it easily could.
Why winter is the danger zone
The sunshine problem peaks in winter, and winter vitamin D deficiency is the predictable result. At higher latitudes, from roughly October to March, the sun sits too low in the sky for its UVB rays to make meaningful vitamin D in the skin at all — so however much time you spend outdoors on a crisp winter's day, your skin simply can't produce it. The body draws on summer stores, but for many people those run down over the colder months, which is why blood levels typically bottom out in late winter and early spring. That is what makes a shortfall so ordinary at this time of year in northern countries: vitamin D in winter comes almost entirely from what you eat or take, not from what your skin makes. It's the single biggest reason health bodies in northern countries advise considering a vitamin D supplement through the darker half of the year.
Vitamin D deficiency signs: what to look for
Deficiency is often quiet, and the vitamin D deficiency symptoms people notice are non-specific — which is exactly why it's so easily missed. Common clues include persistent fatigue and low energy, frequent coughs, colds and infections, bone pain or aching muscles, and low mood (especially over winter). Two more are easier still to miss, because almost nobody connects them to a nutrient: vitamin D deficiency hair loss, meaning gradual thinning rather than sudden shedding, and slow wound healing. Because these overlap with so many other things, they're a prompt to get a simple blood test rather than a diagnosis in themselves — testing is the only way to know your level for sure.
Vitamin D and immunity
The immune connection is one of the better-studied areas, and it's where honest nuance matters most.
A 2017 meta-analysis of individual data from 25 randomised trials (about 11,000 people) found that vitamin D supplementation modestly reduced the risk of acute respiratory infections overall, with the biggest benefit in people who were very deficient to begin with and who took it daily or weekly rather than in large occasional doses (Martineau et al., 2017). A later update tempered this, finding the overall effect smaller and no longer statistically significant — so the fairest reading is a real but modest benefit, concentrated in those who are deficient.
Vitamin D and the brain
Much of the interest in vitamin D brain health comes from observational work, in which low vitamin D has been linked with a higher risk of cognitive decline and dementia. That's genuinely interesting, but observation can't prove cause — and the work on vitamin D and Alzheimer's disease cited below is of exactly that kind. What the research on vitamin D and dementia risk shows, so far, is an association rather than a demonstrated effect.
A large observational study reported that older adults taking vitamin D were significantly less likely to develop dementia over the following years, with the association strongest in women and in those with normal cognition at the outset (Ghahremani et al., 2023). It's an encouraging signal — but because it's observational, and because a randomised trial (the Finnish Vitamin D Trial) found no clear effect on dementia, this remains a promising open question rather than a proven benefit.
Vitamin D and the heart
The evidence on vitamin D heart health takes the same shape: strong observational links that disappoint when they are tested directly. Population data consistently shows a vitamin D deficiency–heart disease link, but the vitamin D cardiovascular trials tell a flatter story. Large trials such as VITAL and the Finnish Vitamin D Trial found no clear reduction in cardiovascular events from supplementing generally healthy, mostly-replete people.
The D-Health trial, one of the largest of its kind (over 21,000 older adults), found that monthly vitamin D produced only a small reduction in major cardiovascular events — the absolute difference was modest, and the authors called for further study, particularly in people taking heart medications (Thompson et al., 2023). The honest summary: correcting deficiency is worthwhile, but vitamin D is not a proven heart-protection pill for people who already have enough.
The partner nutrients: magnesium and K2
Here's something the "just take vitamin D" advice usually misses: vitamin D doesn't work alone. Two partner nutrients matter.
Magnesium activates vitamin D. The enzymes that convert vitamin D into its usable form all depend on magnesium as a cofactor, so if you're low in magnesium, the vitamin D you take may stay largely inactive — which is the practical argument for taking vitamin D and magnesium together rather than treating them as separate errands.
A review concluded that all the enzymes metabolising vitamin D require magnesium, and that ensuring adequate magnesium is necessary to obtain the full benefit of vitamin D — poor magnesium status can leave vitamin D stored but underused (Uwitonze & Razzaque, 2018).
Vitamin K2 is the other half of the picture. Vitamin D increases how much calcium you absorb; K2 then activates the proteins that steer that calcium into your bones and away from your arteries. Taking high-dose vitamin D without enough K2 raises calcium levels without directing traffic — which is why the two are so often paired. We cover this partnership in depth in our guide to vitamin K2.
How to get enough — sensibly
A practical approach beats both neglect and mega-dosing — and for most people in the north the live question is how to get vitamin D in winter, when the skin is out of the game:
- Sunlight, in moderation: short, regular exposure in the warmer months helps build stores, without burning.
- Food: oily fish (salmon, mackerel, sardines), egg yolks, and fortified foods contribute, though diet alone rarely covers a shortfall.
- A supplement, especially in winter or if you're at higher risk: vitamin D3 (cholecalciferol) is the preferred form. It's fat-soluble, so take it with a meal containing some fat, and consider pairing it with magnesium and K2.
- Don't overdo it: vitamin D is fat-soluble and can build up, so very high doses over long periods can cause harm (through raised calcium). More is not better — aim to reach a healthy level, ideally guided by a blood test, rather than taking mega-doses "just in case".
Cover the winter gap — the smart way
Vitamin D3 paired with K2, so the calcium you absorb is directed to your bones. In a liposomal form built for absorption.
Common vitamin D myths — and the vitamin D facts that correct them
Most vitamin D misconceptions start in the same place: a nutrient with real, well-evidenced jobs gets stretched into a cure-all, and the correction then overshoots the other way. The sections above answer most of them — the sunshine and winter picture, and the gap between observational links and trial results for the heart and the brain. Two are worth stating outright.
- "Vitamin D prevents colds." The 2017 meta-analysis found a modest reduction in acute respiratory infections, concentrated in people who were deficient to begin with — but a later update found the overall effect smaller and no longer statistically significant. Modest, uncertain and mostly about correcting a shortfall, in other words, rather than a shield.
- "More is better." It is fat-soluble and accumulates, so the aim is a healthy level confirmed by a blood test, not the highest number you can reach.
Frequently asked questions
Why is vitamin D important, and what does it do?
Its core role is helping you absorb calcium and build strong bones, but as a hormone-like nutrient it also supports immune function, muscles and mood. Severe deficiency causes bone disease; milder shortfalls are linked with a range of problems, though not all of those links are proven to be causal.
How do I know if I'm deficient?
The signs — fatigue, frequent infections, aches, low mood, hair thinning — are non-specific, so the only reliable way is a blood test (25-hydroxyvitamin D). Fatigue is usually the clue people recognise first; vitamin D deficiency fatigue is real enough, but it is also the least specific sign of the lot, so it is worth testing rather than assuming. Deficiency is most likely in winter, in people with darker skin, those who cover up or stay indoors, and older adults.
How much vitamin D from sun exposure do you actually make?
It varies enormously — with latitude, season, time spent outdoors, how much skin is uncovered, skin tone and age. Short, regular exposure in the warmer months builds useful stores for many people, but from roughly October to March at northern latitudes the sun sits too low for the skin to make any at all, however long you stay out.
Can vitamin D prevent dementia, heart disease or colds?
Honestly: partly, and mainly by correcting a deficiency. It modestly lowers respiratory-infection risk (most in deficient people), is linked observationally with lower dementia and heart risk, but large trials have generally not shown it protects already-replete people from dementia or heart disease. Fixing a genuine shortfall is worthwhile; expecting it to work like a drug is not.
Should I take magnesium and K2 with vitamin D?
It's a sensible pairing. Magnesium is needed to activate vitamin D, and K2 directs the calcium vitamin D helps you absorb into bone rather than arteries. Pairing magnesium with vitamin D supplements is straightforward and low-risk, so if you supplement vitamin D, especially at higher doses, it makes sense to ensure good magnesium status and to pair it with K2 as well.
Can you take too much vitamin D?
Yes. Because it's fat-soluble, it accumulates, and sustained very high doses can raise blood calcium to harmful levels. Stick to sensible doses, and if you're taking higher amounts, do it under medical guidance with occasional blood tests rather than guessing.
References
- Martineau AR, Jolliffe DA, Hooper RL, et al. Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data. BMJ. 2017;356:i6583. PMID 28202713
- Uwitonze AM, Razzaque MS. Role of magnesium in vitamin D activation and function. Journal of the American Osteopathic Association. 2018;118(3):181–189. PMID 29480918
- Ghahremani M, Smith EE, Chen HY, Creese B, Goodarzi Z, Ismail Z. Vitamin D supplementation and incident dementia: effects of sex, APOE, and baseline cognitive status. Alzheimer's & Dementia (Amst). 2023;15(1):e12404. PMID 36874594
- Thompson B, Waterhouse M, English DR, et al. Vitamin D supplementation and major cardiovascular events: D-Health randomised controlled trial. BMJ. 2023;381:e075230. PMID 37380191
Read more on the blog
Watch on YouTube
Explore more on our channel, @SANUSqHealthSupplements.
We wish you good health on your terms.
The team at SANUSq.
PS. Curious what's currently on offer? Take a look at our latest promotions.
The health information in this article is provided for educational purposes only. Consult your healthcare professional before making any medical decisions.
Regulatory Notice
The information on our websites, blogs, and emails is provided for informational purposes only and has not been evaluated by the EMA, EFSA, or FDA. It is not intended to replace medical advice provided by your healthcare professional, and is not intended to diagnose, treat, cure, or prevent any disease. Our products are intended for adults aged 18 and over. While the vitamins and supplements mentioned may offer various health benefits, supplements and dietary changes should be considered part of an overall health plan and not a substitute for professional medical treatment. Only a qualified healthcare professional can provide personalized advice and treatment plans based on your individual health needs and medical history; you should consult your healthcare professional before taking any product(s) if you are pregnant or breastfeeding.